The present invention relates to a fractional filtration method and a fractional filtration apparatus and, more particularly, to a fractional filtration method and a fractional filtration apparatus which can separate cuttings from cutting oils through different filtration steps, thereby obtaining cutting oils for different applications, so that the cutting oils can be reused. The cutting oil to be separated is obtained by fractionally filtering a turbid liquid consisting of a liquid and solid material having different particle sizes after the cutting oil is used in various types of industrial machines and which contains cuttings after being used in, e.g., metal cutting.
Conventionally, many filtration apparatuses have been proposed which separate a cutting oil containing cuttings after it is used in metal cutting into cuttings and cutting oils, thereby obtaining cutting oils (hereinafter referred to as cleaned cutting oils) that can be reused.
The "filtration apparatus" of Japanese Patent Publication No. 2-44564 is an example of an improvement of such a fractional filtration apparatus. According to this apparatus, a filtration drum is lightly rotated by causing its one end to float in a cutting oil so as to decrease the weight applied to the: bearing of the filtration drum.
According to the "filtration apparatus" of Japanese Utility Model Application No. 1-31997, floating cuttings which contain air in a continuous helical state during, e.g., lathing, thus obtaining a buoyancy to float on top a cutting oil (to be referred to as a filtration target cutting oil hereinafter) to be filtered which contains cuttings, are caused to settle in the filtration target cutting oil.
When the floating cuttings are left to stand in this state, the floating cuttings accumulate to damage a filter wound on the outer circumferential surface of the filtration drum. This apparatus aims at preventing this damage.
When the cleaned cutting oil cleaned by using one of the above apparatuses is to be used again once or several times, different types of cutting oils may be separately used in accordance with the types of machine tools or the types of cutting operations.
More specifically, when precision cutting is to be performed or when a gun drill or the like is to be used, a new cutting oil or a completely filtered cutting oil which is close to an unused cutting oil is needed. When a cutting oil is used in heavy cutting or rough cutting or is flowed from above to flush cuttings deposited in, e.g., an unmanned factory, into an oil tank, sometimes a completely filtered cutting oil is not always needed.
Where a machining center integrally incorporating various types of machine tools is used, workpieces can be subjected to almost any type of cutting at only one machining center, and the machining center can operate for 24 hours per day. Thus, such a machining center is used in automated and unmanned factories. Various types of lathe turning machines, milling machines, grinding machines, and the like are also used in automated and unmanned factories.
These machine tools require large quantities of cutting oils. Therefore, at least two filtration apparatuses of the type mentioned above, one of which incorporates a filtration drum having a filtration filter of a large mesh size and the other of which incorporates a filtration drum having a filtration filter of a small mesh size, may be installed, and cutting oils for different applications may be obtained by filtration and reused, thereby economizing on running costs.
However, it is uneconomical to install at least two filtration apparatuses, one of which incorporates a filtration drum having a filtration filter of a large mesh size and the other of which incorporates a filtration drum having a filtration filter of a small mesh size, in an automated factory as described above. Also, this requires an additional installation area.
If these filtration apparatuses are disposed independently of various types of machine tools, automation cannot be obtained. Pipes for sending out the cutting oils must be provided between the filtration apparatuses and the various types of machine tools. However, if two or more filtration apparatuses are installed as described above, the piping becomes complicated.
Regarding a conveying unit that recovers used cutting oil and conveys it to the filtration apparatus, if two or more filtration apparatuses are installed, the conveying line of the conveying unit becomes complicated.
As described above, although it is important in an automated factory and the like to economize by reusing cutting oils obtained by filtration for different applications, installation of two or more filtration apparatuses which occupy a large area, compared to the machine tools is not desirable, and requires a higher facility cost, thus making it uneconomical.